Magnetic recording medium

ABSTRACT

The magnetic recording medium includes a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which an isoelectric point of a surface zeta potential of the magnetic layer is equal to or greater than 5.5.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119 to Japanese Patent Application No. 2017-123062 filed on Jun. 23, 2017. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a magnetic recording medium.

2. Description of the Related Art

A magnetic recording medium including a magnetic layer including ferromagnetic powder and a binding agent on a non-magnetic support is called a coating type magnetic recording medium and is widely used as recording medium for various purposes (for example, see JP1997-190623A (JP-H09-190623A).

SUMMARY OF THE INVENTION

In order to continuously or intermittently repeat reproducing a signal recorded on a magnetic recording medium, sliding of a surface of a magnetic layer and a magnetic head is repeated. It is desirable to prevent a decrease in electromagnetic conversion characteristics during such repeated reproducing (hereinafter, also simply referred to as a “decrease in electromagnetic conversion characteristics”), in order to increase reliability of a magnetic recording medium as a recording medium for data storage. This is because a recording medium having a small decrease in electromagnetic conversion characteristics during repeated reproducing can continuously exhibit excellent electromagnetic conversion characteristics, even in a case where the reproducing is continuously or intermittently repeated.

However, as a result of studies of the inventors, it is determined that, a decrease in electromagnetic conversion characteristics during repeated reproducing tends to easily occur in a low temperature and low humidity environment (for example, in an environment of a temperature of 10° C. to 15° C. and relative humidity of approximately 10% to 20%). In recent years, a magnetic recording medium such as a magnetic tape used for data storage is used in a low temperature and low humidity environment such as a data center or the like, in which a temperature and humidity are controlled, in many cases. Therefore, it is desired that the magnetic recording medium has a small decrease in electromagnetic conversion characteristics during repeated reproducing in the low temperature and low humidity environment.

Therefore, an object of the invention is to provide a magnetic recording medium having a small decrease in electromagnetic conversion characteristics, even in a case where the reproducing is repeated in the low temperature and low humidity environment.

According to one aspect of the invention, there is provided a magnetic recording medium comprising: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which an isoelectric point of a surface zeta potential of the magnetic layer is equal to or greater than 5.5.

In one aspect, the isoelectric point may be 5.5 to 7.0.

In one aspect, the binding agent may be a binding agent including an acidic group.

In one aspect, the acidic group may include at least one kind of acidic group selected from the group consisting of a sulfonic acid group and a salt thereof.

In one aspect, the magnetic recording medium may further comprise a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.

In one aspect, the magnetic recording medium may further comprise a back coating layer including non-magnetic powder and a binding agent on a surface side of the non-magnetic support opposite to a surface side provided with the magnetic layer.

According to one aspect of the invention, it is possible to provide a magnetic recording medium having a small decrease in electromagnetic conversion characteristics during repeated reproducing in the low temperature and low humidity environment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

One aspect of the invention relates to a magnetic recording medium including: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, in which an isoelectric point of a surface zeta potential of the magnetic layer is equal to or greater than 5.5.

Hereinafter, the magnetic recording medium will be described more specifically.

Magnetic Layer

Isoelectric Point of Surface Zeta Potential of Magnetic Layer In the magnetic recording medium, the isoelectric point of the surface zeta potential of the magnetic layer is equal to or greater than 5.5. In the invention and the specification, the isoelectric point of the surface zeta potential of the magnetic layer is a value of pH, in a case where a surface zeta potential of the magnetic layer measured by a flow potential method (also referred to as a flow current method) becomes zero. A sample is cut out from the magnetic recording medium which is a measurement target, and the sample is disposed in a measurement cell so that the surface of the magnetic layer which is a target surface for obtaining the surface zeta potential comes into contact with an electrolyte. Pressure in the measurement cell is changed to flow the electrolyte and a flow potential at each pressure is measured, and then, the surface zeta potential is obtained by the following calculation expression.

$\begin{matrix} {\zeta = {\frac{dI}{dp} \times \frac{\eta}{{ɛɛ}_{0}}\frac{L}{A}}} & \left( {{Calculation}\mspace{14mu}{Expression}} \right) \end{matrix}$

[ζ: surface zeta potential, p: pressure, I: flow potential, η: viscosity of electrolyte, ε: relative dielectric constant, ε₀: dielectric constant in a vacuum state, L: length of channel (flow path between two electrodes), A: area of cross section of channel]

The pressure is changed in a range of 0 to 400,000 Pa (0 to 400 mbar). The calculation of the surface zeta potential by flowing the electrolyte to the measurement cell and measuring a flow potential is performed by using electrolytes having different pH (from pH of 9 to pH of 3 at interval of approximately 0.5). A total number of measurement points is 13 from the measurement point of pH 9 to the 13th measurement points of pH 3. By doing so, the surface zeta potentials of each measurement point of pH is obtained. As pH decreases, the surface zeta potential decreases. Thus, two measurement points at which polarity of the surface zeta potential changes (a change from a positive value to a negative value) may appear, while pH decreases from 9 to 3. In a case where such two measurement points appear, pH, in a case where the surface zeta potential is zero, is obtained by interpolation by using a straight line (linear function) showing a relationship between the surface zeta potential and pH of each of the two measurement points. Meanwhile, in a case where all of the surface zeta potentials obtained during the decrease of pH from 9 to 3 is positive value, pH, in a case where the surface zeta potential is zero, is obtained by extrapolation by using a straight line (linear function) showing a relationship between the surface zeta potential and pH of the 13th measurement point (pH of 3) which is the final measurement point and the 12th measurement point. On the other hand, in a case where all of the surface zeta potentials obtained during the decrease of pH from 9 to 3 is negative value, pH, in a case where the surface zeta potential is zero, is obtained by extrapolation by using a straight line (linear function) showing a relationship between the surface zeta potential and pH of the first measurement point (pH of 9) which is the initial measurement point and the 12th measurement point. By doing so, the value of pH, in a case where the surface zeta potential of the magnetic layer measured by the flow potential method is zero, is obtained.

The above measurement is performed three times in total at room temperature by using different samples cut out from the same magnetic recording medium (magnetic recording medium which is a measurement target), and pH, in a case where the surface zeta potential of each sample is zero, is obtained. For the viscosity and the relative dielectric constant of the electrolyte, a measurement value at room temperature is used. The room temperature is set as 20° C. to 27° C. An arithmetical mean of three pHs obtained as described above is an isoelectric point of the surface zeta potential of the magnetic layer of the magnetic recording medium which is a measurement target. In addition, as the electrolyte having pH of 9, an electrolyte obtained by adjusting pH of a KCl aqueous solution having a concentration of 1 mmol/L to 9 by using a KOH aqueous solution having a concentration of 0.1 mol/L is used. As the electrolyte having other pH, an electrolyte obtained by adjusting pH of the electrolyte having pH of 9, which is adjusted as described above, by using an HCl aqueous solution having a concentration of 0.1 mol/L is used.

The isoelectric point of the surface zeta potential measured by the method described above is an isoelectric point obtained regarding the surface of the magnetic layer, unlike an isoelectric point of ferromagnetic powder included in a magnetic layer disclosed in JP1997-190623A (JP-H09-190623A), for example. As a result of the intensive studies, the inventors have newly found that, by setting the isoelectric point of the surface zeta potential of the magnetic layer to be equal to or greater than 5.5, it is possible to prevent a decrease in electromagnetic conversion characteristics during repeated reproducing in a low temperature and low humidity environment. As a reason thereof, the inventors have thought that, in a magnetic recording medium in which an isoelectric point of a surface zeta potential of a magnetic layer is in a neutral to basic pH region, the surface of the magnetic layer and a magnetic head hardly react with each other electrochemically, and scraps generated due to chipping of the surface of the magnetic layer due to contact between the surface of the magnetic layer and the magnetic head are hardly stuck to the magnetic head. However, it is merely a surmise. Thus, the invention is not limited to the above surmise.

As will be described later in detail, the isoelectric point of the surface zeta potential of the magnetic layer can be controlled by the kind of a component used for forming the magnetic layer, a formation step of the magnetic layer, and the like. From a viewpoint of ease of controlling, the isoelectric point of the surface zeta potential of the magnetic layer is preferably equal to or smaller than 7.0, more preferably equal to or smaller than 6.7, and even more preferably equal to or smaller than 6.5. In addition, the isoelectric point of the surface zeta potential of the magnetic layer is equal to or greater than 5.5, preferably equal to or greater than 5.7, and more preferably equal to or greater than 6.0.

Next, the magnetic layer will be described more specifically.

Ferromagnetic Powder

As the ferromagnetic powder included in the magnetic layer, ferromagnetic powder normally used in the magnetic layer of various magnetic recording media can be used. It is preferable to use ferromagnetic powder having a small average particle size, from a viewpoint of improvement of recording density of the magnetic recording medium. From this viewpoint, ferromagnetic powder having an average particle size equal to or smaller than 50 nm is preferably used as the ferromagnetic powder. Meanwhile, the average particle size of the ferromagnetic powder is preferably equal to or greater than 10 nm, from a viewpoint of stability of magnetization.

As a preferred specific example of the ferromagnetic powder, ferromagnetic hexagonal ferrite powder can be used. An average particle size of the ferromagnetic hexagonal ferrite powder is preferably 10 nm to 50 nm and more preferably 20 nm to 50 nm, from a viewpoint of improvement of recording density and stability of magnetization. For details of the ferromagnetic hexagonal ferrite powder, descriptions disclosed in paragraphs 0012 to 0030 of JP2011-225417A, paragraphs 0134 to 0136 of JP2011-216149A, and paragraphs 0013 to 0030 of JP2012-204726A can be referred to, for example.

As a preferred specific example of the ferromagnetic powder, ferromagnetic metal powder can also be used. An average particle size of the ferromagnetic metal powder is preferably 10 nm to 50 nm and more preferably 20 nm to 50 nm, from a viewpoint of improvement of recording density and stability of magnetization. For details of the ferromagnetic metal powder, descriptions disclosed in paragraphs 0137 to 0141 of JP2011-216149A and paragraphs 0009 to 0023 of JP2005-251351A can be referred to, for example.

In the invention and the specification, average particle sizes of various powder such as the ferromagnetic powder and the like are values measured by the following method with a transmission electron microscope, unless otherwise noted.

The powder is imaged at a magnification ratio of 100,000 with a transmission electron microscope, the image is printed on photographic printing paper so that the total magnification of 500,000 to obtain an image of particles configuring the powder. A target particle is selected from the obtained image of particles, an outline of the particle is traced with a digitizer, and a size of the particle (primary particle) is measured. The primary particle is an independent particle which is not aggregated.

The measurement described above is performed regarding 500 particles randomly extracted. An arithmetical mean of the particle size of 500 particles obtained as described above is an average particle size of the powder. As the transmission electron microscope, a transmission electron microscope H-9000 manufactured by Hitachi, Ltd. can be used, for example. In addition, the measurement of the particle size can be performed by well-known image analysis software, for example, image analysis software KS-400 manufactured by Carl Zeiss. The average particle size shown in examples which will be described later is a value measured by using transmission electron microscope H-9000 manufactured by Hitachi, Ltd. as the transmission electron microscope, and image analysis software KS-400 manufactured by Carl Zeiss as the image analysis software, unless otherwise noted. In the invention and the specification, the powder means an aggregate of a plurality of particles. For example, the ferromagnetic powder means an aggregate of a plurality of ferromagnetic particles. The aggregate of the plurality of particles not only includes an aspect in which particles configuring the aggregate are directly in contact with each other, but also includes an aspect in which a binding agent or an additive which will be described later is interposed between the particles. A term “particles” is also used for describing the powder.

As a method of collecting a sample powder from the magnetic recording medium in order to measure the particle size, a method disclosed in a paragraph of 0015 of JP2011-048878A can be used, for example.

In the invention and the specification, unless otherwise noted, (1) in a case where the shape of the particle observed in the particle image described above is a needle shape, a fusiform shape, or a columnar shape (here, a height is greater than a maximum long diameter of a bottom surface), the size (particle size) of the particles configuring the powder is shown as a length of a long axis configuring the particle, that is, a long axis length, (2) in a case where the shape of the particle is a planar shape or a columnar shape (here, a thickness or a height is smaller than a maximum long diameter of a plate surface or a bottom surface), the particle size is shown as a maximum long diameter of the plate surface or the bottom surface, and (3) in a case where the shape of the particle is a sphere shape, a polyhedron shape, or an unspecified shape, and the long axis configuring the particles cannot be specified from the shape, the particle size is shown as an equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.

In addition, regarding an average acicular ratio of the powder, a length of a short axis, that is, a short axis length of the particles is measured in the measurement described above, a value of (long axis length/short axis length) of each particle is obtained, and an arithmetical mean of the values obtained regarding 500 particles is calculated. Here, unless otherwise noted, in a case of (1), the short axis length as the definition of the particle size is a length of a short axis configuring the particle, in a case of (2), the short axis length is a thickness or a height, and in a case of (3), the long axis and the short axis are not distinguished, thus, the value of (long axis length/short axis length) is assumed as 1, for convenience.

In addition, unless otherwise noted, in a case where the shape of the particle is specified, for example, in a case of definition of the particle size (1), the average particle size is an average long axis length, in a case of the definition (2), the average particle size is an average plate diameter. In a case of the definition (3), the average particle size is an average diameter (also referred to as an average particle diameter).

The content (filling percentage) of the ferromagnetic powder of the magnetic layer is preferably 50% to 90% by mass and more preferably 60% to 90% by mass. The components other than the ferromagnetic powder of the magnetic layer are at least a binding agent and one or more kinds of additives may be randomly included. A high filling percentage of the ferromagnetic powder in the magnetic layer is preferable from a viewpoint of improvement recording density.

Binding Agent and Curing Agent

The magnetic recording medium is a coating type magnetic recording medium and includes a binding agent in the magnetic layer. The binding agent is one or more kinds of resin. As the binding agent, various resins normally used as a binding agent of a coating type magnetic recording medium can be used. For example, as the binding agent, a resin selected from a polyurethane resin, a polyester resin, a polyamide resin, a vinyl chloride resin, an acrylic resin obtained by copolymerizing styrene, acrylonitrile, or methyl methacrylate, a cellulose resin such as nitrocellulose, an epoxy resin, a phenoxy resin, and a polyvinylalkylal resin such as polyvinyl acetal or polyvinyl butyral can be used alone or a plurality of resins can be mixed with each other to be used. Among these, a polyurethane resin, an acrylic resin, a cellulose resin, and a vinyl chloride resin are preferable. These resins may be homopolymers or copolymers. These resins can be used as the binding agent even in the non-magnetic layer and/or a back coating layer which will be described later.

For the binding agent described above, description disclosed in paragraphs 0028 to 0031 of JP2010-24113A can be referred to. An average molecular weight of the resin used as the binding agent can be, for example, 10,000 to 200,000 as a weight-average molecular weight. The weight-average molecular weight of the invention and the specification is a value obtained by performing polystyrene conversion of a value measured by gel permeation chromatography (GPC) under the following measurement conditions. The weight-average molecular weight of the binding agent shown in examples which will be described later is a value obtained by performing polystyrene conversion of a value measured under the following measurement conditions.

GPC device: HLC-8120 (manufactured by Tosoh Corporation)

Column: TSK gel Multipore HXL-M (manufactured by Tosoh Corporation, 7.8 mmlD (inner diameter)×30.0 cm)

Eluent: Tetrahydrofuran (THF)

In one aspect, as the binding agent, a binding agent including an acidic group can be used. The acidic group of the invention and the specification is used as a meaning including a state of a group capable of emitting H⁺ in water or a solvent including water (aqueous solvent) to dissociate anions and a salt thereof. Specific examples of the acidic group include a sulfonic acid group, a sulfate group, a carboxyl group, a phosphate group, and a salt thereof. For example, a salt of sulfonic acid group (—SO3H) is represented by —SO3M, and M represents a group representing an atom (for example, alkali metal atom or the like) which may be cations in water or in an aqueous solvent. The same applies to aspects of salts of various groups described above. As an example of the binding agent including the acidic group, a resin including at least one kind of acidic group selected from the group consisting of a sulfonic acid group and a salt thereof (for example, a polyurethane resin or a vinyl chloride resin) can be used. However, the resin included in the magnetic layer is not limited to these resins. In addition, in the binding agent including the acidic group, a content of the acidic group can be, for example, 0.03 to 0.50 meq/g. The content of various functional groups such as the acidic group included in the resin can be obtained by a well-known method in accordance with the kind of the functional group. The amount of the binding agent used in a magnetic layer forming composition can be, for example, 1.0 to 30.0 parts by mass with respect to 100.0 parts by mass of the ferromagnetic powder.

In regards to the controlling of the isoelectric point of the surface zeta potential of the magnetic layer, the inventors have surmised that formation of the magnetic layer so that the amount of the acidic component present in the surface portion of the magnetic layer decreases contributes to an increase in value of the isoelectric point. In addition, the inventors have surmised that an increase in amount of the basic component present in the surface portion of the magnetic layer also contributes to an increase in value of the isoelectric point. The acidic component is used as a meaning including a state of a component capable of emitting H⁺ in water or an aqueous solvent to dissociate anions and a salt thereof. The basic component is used as a meaning including a state of a component capable of emitting OH⁻ in water or an aqueous solvent to dissociate cations and a salt thereof. For example, it is thought that, in a case of using the acidic component, performing a process of unevenly distributing the acidic component in a surfaces portion of a coating layer of a magnetic layer forming composition first, and then, performing a process of decreasing the amount of acidic component in the surface portion contribute to an increase in value of the isoelectric point of the surface zeta potential of the magnetic layer to control the isoelectric point to be equal to or greater than 5.5. For example, the inventors have thought that, in a step of applying a magnetic layer forming composition onto a non-magnetic support directly or through a non-magnetic layer, the applying which is performed in an alternating magnetic field by applying an alternating magnetic field contributes to uneven distribution of the acidic component in the surface portion of the coating layer of the magnetic layer forming composition. In addition, the inventors have surmised that a burnishing process performed subsequent thereto contributes to removal of at least some acidic component unevenly distributed. The burnishing process is a process of rubbing a surface of a process target with a member (for example, abrasive tape or a grinding tool such as a blade for grinding or a wheel for grinding). A magnetic layer forming step including the burnishing process will be described in detail. As the acidic component, for example, a binding agent including an acidic group can be used.

In addition, a curing agent can also be used together with the resin which can be used as the binding agent. As the curing agent, in one aspect, a thermosetting compound which is a compound in which a curing reaction (crosslinking reaction) proceeds due to heating can be used, and in another aspect, a photocurable compound in which a curing reaction (crosslinking reaction) proceeds due to light irradiation can be used. At least a part of the curing agent is included in the magnetic layer in a state of being reacted (crosslinked) with other components such as the binding agent, by proceeding the curing reaction in the magnetic layer forming step. This point is the same as regarding a layer formed by using a composition, in a case where the composition used for forming the other layer includes the curing agent. The preferred curing agent is a thermosetting compound, polyisocyanate is suitable. For details of the polyisocyanate, descriptions disclosed in paragraphs 0124 and 0125 of JP2011-216149A can be referred to, for example. The amount of the curing agent can be, for example, 0 to 80.0 parts by mass with respect to 100.0 parts by mass of the binding agent in the magnetic layer forming composition, and is preferably 50.0 to 80.0 parts by mass, from a viewpoint of improvement of hardness of the magnetic layer.

Additives

The magnetic layer includes ferromagnetic powder and the binding agent, and may include one or more kinds of additives, if necessary. As the additives, the curing agent described above is used as an example. In addition, examples of the additive included in the magnetic layer include non-magnetic powder (for example, inorganic powder or carbon black), a lubricant, a dispersing agent, a dispersing assistant, an antibacterial agent, an antistatic agent, and an antioxidant. As the non-magnetic powder, non-magnetic powder which can function as an abrasive, non-magnetic powder (for example, non-magnetic colloid particles) which can function as a projection formation agent which forms projections suitably protruded from the surface of the magnetic layer, and the like can be used. An average particle size of colloidal silica (silica colloid particles) shown in the examples which will be described later is a value obtained by a method disclosed in a measurement method of an average particle diameter in a paragraph 0015 of JP2011-048878A. As the additives, a commercially available product can be suitably selected according to the desired properties or manufactured by a well-known method, and can be used with any amount. As an example of the additive which can be used in the magnetic layer including the abrasive, a dispersing agent disclosed in paragraphs 0012 to 0022 of JP2013-131285A can be used as a dispersing agent for improving dispersibility of the abrasive.

The magnetic layer described above can be provided on the surface of the non-magnetic support directly or indirectly through the non-magnetic layer.

Non-Magnetic Layer

Next, the non-magnetic layer will be described. The magnetic recording medium may include a magnetic layer directly on the surface of the non-magnetic support or may include a magnetic layer on the surface of the non-magnetic support directly or indirectly through the non-magnetic layer including the non-magnetic powder and the binding agent. The non-magnetic powder used in the non-magnetic layer may be inorganic powder or organic powder. In addition, carbon black and the like can be used. Examples of the inorganic powder include powder of metal, metal oxide, metal carbonate, metal sulfate, metal nitride, metal carbide, and metal sulfide. These non-magnetic powder can be purchased as a commercially available product or can be manufactured by a well-known method. For details thereof, descriptions disclosed in paragraphs 0146 to 0150 of JP2011-216149A can be referred to. For carbon black which can be used in the non-magnetic layer, descriptions disclosed in paragraphs 0040 and 0041 of JP2010-24113A can be referred to. The content (filling percentage) of the non-magnetic powder of the non-magnetic layer is preferably 50% to 90% by mass and more preferably 60% to 90% by mass.

In regards to other details of a binding agent or additives of the non-magnetic layer, the well-known technology regarding the non-magnetic layer can be applied. In addition, in regards to the type and the content of the binding agent, and the type and the content of the additive, for example, the well-known technology regarding the magnetic layer can be applied.

In the invention and the specification, the non-magnetic layer also includes a substantially non-magnetic layer including a small amount of ferromagnetic powder as impurities or intentionally, together with the non-magnetic powder. Here, the substantially non-magnetic layer is a layer having a residual magnetic flux density equal to or smaller than 10 mT, a layer having coercivity equal to or smaller than 7.96 kA/m(100 Oe), or a layer having a residual magnetic flux density equal to or smaller than 10 mT and coercivity equal to or smaller than 7.96 kA/m(100 Oe). It is preferable that the non-magnetic layer does not have a residual magnetic flux density and coercivity.

Non-Magnetic Support

Next, the non-magnetic support (hereinafter, also simply referred to as a “support”) will be described. As the non-magnetic support, well-known components such as polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamide imide, aromatic polyamide subjected to biaxial stretching are used. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable. Corona discharge, plasma treatment, easy-bonding treatment, or heat treatment may be performed with respect to these supports in advance.

Back Coating Layer

The magnetic recording medium can also include a back coating layer including non-magnetic powder and a binding agent on a surface side of the non-magnetic support opposite to the surface side provided with the magnetic layer. The back coating layer preferably includes any one or both of carbon black and inorganic powder. In regards to the binding agent included in the back coating layer and various additives which can be randomly included therein, a well-known technology regarding the treatment of the magnetic layer and/or the non-magnetic layer can be applied.

Various Thicknesses

A thickness of the non-magnetic support is preferably 3.00 to 20.00 μm, more preferably 3.00 to 10.00 μm, and even more preferably 3.00 to 6.00 μm.

A thickness of the magnetic layer can be optimized according to a saturation magnetization amount of a magnetic head used, a head gap length, a recording signal band, and the like. The thickness of the magnetic layer is normally 0.01 μm to 0.15 μm, and is preferably 0.02 μm to 0.12 μm and more preferably 0.03 μm to 0.10 μm from a viewpoint of realization of high-density recording. The magnetic layer may be at least one layer, or the magnetic layer can be separated to two or more layers having magnetic properties, and a configuration regarding a well-known multilayered magnetic layer can be applied. A thickness of the magnetic layer which is separated into two or more layers is a total thickness of the layers.

A thickness of the non-magnetic layer is, for example, 0.10 to 1.50 μm and is preferably 0.10 to 1.00 μm.

A thickness of the back coating layer is preferably equal to or smaller than 0.90 μm and even more preferably 0.10 to 0.70 μm.

The thicknesses of various layers of the magnetic recording medium and the non-magnetic support can be acquired by a well-known film thickness measurement method. As an example, a cross section of the magnetic recording medium in a thickness direction is, for example, exposed by a well-known method of ion beams or microtome, and the exposed cross section is observed with a scanning electron microscope. In the cross section observation, various thicknesses can be acquired as a thickness acquired at one portion of the cross section in the thickness direction, or an arithmetical mean of thicknesses acquired at a plurality of portions of two or more portions, for example, two portions which are randomly extracted. In addition, the thickness of each layer may be acquired as a designed thickness calculated according to the manufacturing conditions.

Manufacturing Method of Magnetic Recording Medium

Each composition for forming the magnetic layer, and the non-magnetic layer and the back coating layer which are randomly provided, normally includes a solvent, together with various components described above. As the solvent, various organic solvents generally used for manufacturing a coating type magnetic recording medium can be used. The amount of the solvent in each layer forming composition is not particularly limited, and can be set to be the same as that of each layer forming composition of a typical coating type magnetic recording medium. Steps of preparing the composition for forming each layer generally include at least a kneading step, a dispersing step, and a mixing step provided before and after these steps, if necessary. Each step may be divided into two or more stages. All of raw materials used in the invention may be added at an initial stage or in a middle stage of each step. In addition, each raw material may be separately added in two or more steps.

In order to prepare each layer forming composition, a well-known technology can be used. In the kneading step, an open kneader, a continuous kneader, a pressure kneader, or a kneader having a strong kneading force such as an extruder is preferably used. The details of the kneading processes of these kneaders are disclosed in JP1989-106338A (JP-H01-106338A) and JP1989-79274A (JP-H01-79274A). In addition, in order to disperse each layer forming composition, as a dispersion medium, at least one or more kinds of dispersion beads selected from the group consisting of glass beads and other dispersion beads can be used. As such dispersion beads, zirconia beads, titania beads, and steel beads which are dispersion beads having high specific gravity are suitable. These dispersion beads are preferably used by optimizing a particle diameter (bead diameter) and a filling percentage. As a dispersing machine, a well-known dispersing machine can be used. Each layer forming composition may be filtered by a well-known method before performing the coating step. The filtering can be performed by using a filter, for example. As the filter used in the filtering, a filter having a hole diameter of 0.01 to 3 μm (for example, filter made of glass fiber or filter made of polypropylene) can be used, for example.

The magnetic layer can be formed by directly applying and drying the magnetic layer forming composition onto the surface of the non-magnetic support or performing multilayer coating with the non-magnetic layer forming composition in order or at the same time and drying. For details of the coating for forming each layer, a description disclosed in a paragraph 0066 of JP2010-231843A can be referred to. In addition, the coating of the magnetic layer forming composition performed in an alternating magnetic field can contribute to the controlling of the isoelectric point of a surface zeta potential of the magnetic layer to be equal to or greater than 5.5. The inventors have surmised that this is because, an acidic component (for example, the binding agent including an acidic group) is easily unevenly distributed to a surface portion of a coating layer of the magnetic layer forming composition due to the applied alternating magnetic field, and thus, by drying this coating layer, a magnetic layer in which the acidic component is unevenly distributed to the surface portion is obtained. In addition, the inventors have surmised that the burnishing process performed subsequent thereto contributes to removal of at least some acidic component unevenly distributed to control the isoelectric point of a surface zeta potential of the magnetic layer to be equal to or greater than 5.5. However, this is merely a surmise. The applying of the alternating magnetic field can be performed by disposing a magnet in a coating device so that the alternating magnetic field is applied vertically to the surface of the coating layer of the magnetic layer forming composition. A magnetic field strength of the alternating magnetic field can be, for example, set as approximately 0.05 to 3.00 T. However, there is no limitation to this range. The “vertical” in the invention and the specification does not mean only a vertical direction in the strict sense, but also includes a range of errors allowed in the technical field of the invention. For example, the range of errors means a range of less than ±10° from an exact vertical direction.

The burnishing process is a process of rubbing a surface of a process target with a member (for example, abrasive tape or a grinding tool such as a blade for grinding or a wheel for grinding) and can be performed in the same manner as a well-known burnishing process for manufacturing a coating type magnetic recording medium. The burnishing process can be preferably performed by performing one or both of rubbing (polishing) of a surface of a coating layer of a process target with an abrasive tape, and rubbing (grinding) of a surface of a coating layer of a process target with a grinding tool. As the abrasive tape, a commercially available product may be used or an abrasive tape manufactured by a well-known method may be used. In addition, as the grinding tool, a well-known blade for grinding such as a fixed type blade, a diamond wheel, or a rotary blade, or a wheel for grinding can be used. Further, a wiping process of wiping the surface of the coating layer rubbed with the abrasive tape and/or the grinding tool with a wiping material may be performed. For details of the preferable abrasive tape, grinding tool, burnishing process, and wiping process, paragraphs 0034 to 0048, FIG. 1, and examples of JP-H06-52544A can be referred to. It is thought that, as the burnishing process is reinforced, it is possible remove a large amount of the acidic component unevenly distributed to the surface portion of the coating layer of the magnetic layer forming composition by performing the applying in the alternating magnetic field. As an abrasive having high hardness is used as an abrasive included in the abrasive tape, the burnishing process can be reinforced, and as the amount of the abrasive in the abrasive tape increases, the burnishing process can be reinforced. In addition, as a grinding tool having high hardness is used as the grinding tool, the burnishing process can be reinforced. In regards to burnishing process conditions, as a sliding speed of the surface of the coating layer of the process target and the member (for example, the abrasive tape or the grinding tool) increases, the burnishing process can be reinforced. The sliding speed can be increased by increasing one or both of a speed of movement of the member and a speed of movement of the magnetic tape of the process target. Although the reason is not clear, as the amount of the binding agent including the acidic group in the coating layer of the magnetic layer forming composition is great, the isoelectric point of a surface zeta potential of the magnetic layer tends to increase after the burnishing process.

In a case where the magnetic layer forming composition includes a curing agent, a curing process is preferably performed in any stage of the step for forming the magnetic layer. The burnishing process is preferably performed at least before the curing process. After the curing process, the burnishing process may be further performed. The inventors have thought that it is preferable to perform the burnishing process before the curing process, in order to increase removal efficiency for removing the acidic component from the surface portion of the coating layer of the magnetic layer forming composition. The curing process can be performed by a process of a heat treatment or light irradiation, according to the kind of the curing agent included in the magnetic layer forming composition. The curing process conditions are not particularly limited and may be suitably set according to the list of the magnetic layer forming composition, the kind of the curing agent, the thickness of the coating layer, and the like. For example, in a case where the coating layer is formed by using the magnetic layer forming composition including polyisocyanate as the curing agent, the curing process is preferably a heat treatment. In a case where the curing agent is included in a layer other than the coating layer of the magnetic layer forming composition, the curing process can proceed with the curing reaction of the layer. Alternatively, the curing process can also be performed separately.

The surface smoothing treatment can be preferably performed before the curing process. The surface smoothing treatment is a process performed for increasing smoothness of the surface of the magnetic layer and/or the surface of the back coating layer and is preferably performed by a calender process. For details of the calender process, description disclosed in a paragraph 0026 of JP2010-231843A can be referred to, for example.

For various other steps for manufacturing the magnetic recording medium, a well-known technology can be applied. For details of the various steps, descriptions disclosed in paragraphs 0067 to 0070 of JP2010-231843A can be referred to, for example.

As described above, it is possible to obtain the magnetic recording medium according to one aspect of the invention. The magnetic recording medium can be a tape-shaped magnetic recording medium (magnetic tape) or can also be a disk-shaped magnetic recording medium (magnetic disk). For example, the magnetic tape is normally accommodated in a magnetic tape cartridge and the magnetic tape cartridge is mounted in a magnetic tape device (referred to as a drive). A servo pattern can also be formed in the magnetic tape by a well-known method, in order to allow head tracking servo to be performed in the drive. It is possible to prevent a decrease in electromagnetic conversion characteristics, in a case of repeatedly reproducing information recorded on a magnetic tape in a drive in the low temperature and low humidity environment, as long as it is the magnetic recording medium according to one aspect of the invention.

EXAMPLES

Hereinafter, the invention will be described with reference to examples. However, the invention is not limited to aspects shown in the examples. “Parts” and “%” in the following description are based on mass.

A “binding agent A” described below is a SO₃Na group-containing polyurethane resin (weight-average molecular weight: 70,000, SO₃Na group: 0.20 meq/g).

A “binding agent B” described below is a vinyl chloride copolymer (product name: MR110, SO₃K group-containing vinyl chloride copolymer, SO₃K group: 0.07 meq/g) manufactured by Kaneka Corporation.

Manufacturing of Magnetic Tape

Example 1

(1) Preparation of Alumina Dispersion

3.0 parts of 2,3-dihydroxynaphthalene (manufactured by Tokyo Chemical Industry Co., Ltd.), 31.3 parts of a 32% solution (solvent is a mixed solvent of methyl ethyl ketone and toluene) of a SO₃Na group-containing polyester polyurethane resin (UR-4800 (SO₃Na group: 0.08 meq/g) manufactured by Toyobo Co., Ltd.), and 570.0 parts of a mixed solvent of methyl ethyl ketone and cyclohexanone (mass ratio of 1:1) as a solvent were mixed with 100.0 parts of alumina powder (HIT-80 manufactured by Sumitomo Chemical Co., Ltd.) having a gelatinization ratio of 65% and a Brunauer-Emmett-Teller (BET) specific surface area of 20 m²/g, and dispersed in the presence of zirconia beads by a paint shaker for 5 hours. After the dispersion, the dispersion liquid and the beads were separated by a mesh and an alumina dispersion was obtained.

(2) Magnetic Layer Forming Composition List

Magnetic Liquid

Ferromagnetic powder: 100.0 parts

-   -   Ferromagnetic hexagonal barium ferrite powder having average         particle size (average plate diameter) of 21 nm

Binding agent (see Table 1): see Table 1

Cyclohexanone: 150.0 parts

Methyl ethyl ketone: 150.0 parts

Abrasive Solution

Alumina dispersion prepared in the section (1): 6.0 parts

Silica Sol (projection forming agent liquid)

Colloidal silica (Average particle size: 120 nm) 2.0 parts

Methyl ethyl ketone: 1.4 parts

Other Components

Stearic acid: 2.0 parts

Stearic acid amide: 0.2 parts

Butyl stearate: 2.0 parts

Polyisocyanate (CORONATE (registered trademark) manufactured by Tosoh Corporation): 2.5 parts

Finishing Additive Solvent

Cyclohexanone: 200.0 parts

Methyl ethyl ketone: 200.0 parts

(3) Non-Magnetic Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 100.0 parts

-   -   Average particle size (average long axis length): 0.15 μm     -   Average acicular ratio: 7     -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

Binding agent A: 18.0 parts

Stearic acid: 2.0 parts

Stearic acid amide: 0.2 parts

Butyl stearate: 2.0 parts

Cyclohexanone: 300.0 parts

Methyl ethyl ketone: 300.0 parts

(4) Back Coating Layer Forming Composition List

Non-magnetic inorganic powder: α-iron oxide: 80.0 parts

-   -   Average particle size (average long axis length): 0.15 μm     -   Average acicular ratio: 7     -   BET specific surface area: 52 m²/g

Carbon black: 20.0 parts

-   -   Average particle size: 20 nm

A vinyl chloride copolymer: 13.0 parts

Sulfonic acid group-containing polyurethane resin: 6.0 parts

Phenylphosphonic acid: 3.0 parts

Methyl ethyl ketone: 155.0 parts

Polyisocyanate: 5.0 parts

Cyclohexanone: 355.0 parts

(5) Preparation of Each Layer Forming Composition

The magnetic layer forming composition was prepared by the following method.

The magnetic liquid was prepared by dispersing (beads-dispersing) each component by using a batch type vertical sand mill for 24 hours. Zirconia beads having a bead diameter of 0.5 mm were used as the dispersion beads.

The prepared magnetic liquid and abrasive solution, and other components (silica sol, other components, and finishing additive solvent) were mixed with each other and beads-dispersed for 5 minutes by using the sand mill, and the treatment (ultrasonic dispersion) was performed with a batch type ultrasonic device (20 kHz, 300 W) for 0.5 minutes. After that, the obtained mixed solution was filtered by using a filter having a hole diameter of 0.5 μm, and the magnetic layer forming composition was prepared.

The non-magnetic layer forming composition was prepared by the following method.

Each component excluding the lubricant (stearic acid, stearic acid amide, and butyl stearate), cyclohexanone, and methyl ethyl ketone was dispersed by using batch type vertical sand mill for 24 hours to obtain a dispersion liquid. As the dispersion beads, zirconia beads having a bead diameter of 0.5 mm were used. After that, the remaining components were added into the obtained dispersion liquid and stirred with a dissolver. The dispersion liquid obtained as described above was filtered with a filter having a hole diameter of 0.5 μm and a non-magnetic layer forming composition was prepared.

The back coating layer forming composition was prepared by the following method.

Each component excluding polyisocyanate and cyclohexanone was kneaded by an open kneader and diluted, and was subjected to a dispersion process of 12 passes, with a transverse beads mill disperser and zirconia beads having a bead diameter of 1 mm, by setting a bead filling percentage as 80 volume %, a circumferential speed of rotor distal end as 10 m/sec, and a retention time for 1 pass as 2 minutes. After that, the remaining components were added into the obtained dispersion liquid and stirred with a dissolver. The dispersion liquid obtained as described above was filtered with a filter having a hole diameter of 1 μm and a back coating layer forming composition was prepared.

(6) Manufacturing Method of Magnetic Tape

The non-magnetic layer forming composition prepared in the section (5) was applied to a surface of a support made of polyethylene naphthalate having a thickness of 5.00 μm so that the thickness after the drying becomes 1.00 μm and was dried to form a non-magnetic layer.

Then, in a coating device disposed with a magnet for applying an alternating magnetic field, the magnetic layer forming composition prepared in the section (5) was applied onto the surface of the non-magnetic layer so that the thickness after the drying becomes 0.10 μm, while applying an alternating magnetic field (magnetic field strength: 0.15 T), to form a coating layer. The applying of the alternating magnetic field was performed so that the alternating magnetic field was applied vertically to the surface of the coating layer. After that, a homeotropic alignment process was performed by applying a magnetic field having a magnetic field strength of 0.30 T in a vertical direction with respect to a surface of a coating layer, while the coating layer of the magnetic layer forming composition is wet (not dried). After that, the coating layer was dried to form a magnetic layer.

After that, the back coating layer forming composition prepared in the section (5) was applied to the surface of the support made of polyethylene naphthalate on a side opposite to the surface where the non-magnetic layer and the magnetic layer were formed, so that the thickness after the drying becomes 0.50 μm, and was dried to form a back coating layer.

The magnetic tape obtained as described above was slit to have a width of ½ inches (0.0127 meters), and the burnishing process and the wiping process of the surface of the coating layer of the magnetic layer forming composition were performed. The burnishing process and the wiping process were performed in a process device having a configuration shown in FIG. 1 of JP-H-06-52544A, by using a commercially available abrasive tape (product name: MA22000 manufactured by Fujifilm Holdings Corporation, abrasive: diamond/Cr₂O₃/red oxide) as an abrasive tape, by using a commercially available sapphire diamond (manufactured by Kyocera Corporation, width of 5 mm, length of 35 mm, an angle of a distal end of 60 degrees) as a blade for grinding, and by using a commercially available wiping material (product name: WRP736 manufactured by Kuraray Co., Ltd.) as a wiping material. For the process conditions, process conditions of Example 12 of JP-H06-52544A were used.

After the burnishing process and the wiping process, a calender process (surface smoothing treatment) was performed by using a calender roll configured of only a metal roll, at a speed of 80 m/min, linear pressure of 300 kg/cm (294 kN/m), and a calender temperature (surface temperature of a calender roll) of 100° C.

Then, the heat treatment (curing process) was performed in the environment of the atmosphere temperature of 70° C. for 36 hours. After that, a servo pattern was formed on the magnetic layer by a commercially available servo writer.

By doing so, a magnetic tape of Example 1 was manufactured.

Examples 2 to 6 and Comparative Examples 1 to 8

A magnetic tape was manufactured by the same method as in Example, except that various conditions were changed as shown in Table 1.

In Table 1, in Examples 2 to 6 in which “performed” is shown in the column of the alternating magnetic field application during coating and the column of the burnishing process, the step subsequent to the coating step of the magnetic layer forming composition was performed by the same method as in Example 1. That is, the application of the alternating magnetic field was performed during coating of the magnetic layer forming composition in the same manner as in Example 1, and the burnishing process and the wiping process were performed with respect to the coating layer of the magnetic layer forming composition.

With respect to this, in Comparative Example 5 in which “not performed” is shown in the column of the burnishing process, the step subsequent to the coating step of the magnetic layer forming composition was performed by the same method as in Example 1, except that the burnishing process and the wiping process were not performed with respect to the coating layer of the magnetic layer forming composition.

In Comparative Example 6 in which “not performed” is shown in the column of the alternating magnetic field application during coating, the step subsequent to the coating step of the magnetic layer forming composition was performed by the same method as in Example 1, except that the application of the alternating magnetic field was not performed.

In Comparative Examples 1 to 4, 7, and 8 in which “not performed” is shown in the column of the alternating magnetic field application during coating and the column of the burnishing process, the step subsequent to the coating step of the magnetic layer forming composition was performed by the same method as in Example 1, except that application of the alternating magnetic field is not performed and the burnishing process and the wiping process were not performed with respect to the coating layer of the magnetic layer forming composition.

Evaluation of Physical Properties of Magnetic Tape

(1) Isoelectric Point of Surface Zeta Potential of Magnetic Layer

Six samples for isoelectric point measurement were cut out from each magnetic tape of the examples and the comparative examples and disposed in the measurement cell of two samples in one measurement. In the measurement cell, a sample installing surface and a surface of the back coating layer of the sample were bonded to each other by using a double-sided tape in upper and lower sample table (size of each sample installing surface is 1 cm×2 cm) of the measurement cell. Accordingly, in a case where an electrolyte flows in the measurement cell, the surface of the magnetic layer of the sample comes into contact with the electrolyte, and thus, the surface zeta potential of the magnetic layer can be measured. The measurement was performed three times in total by using two samples in each measurement, and the isoelectric points of the surface zeta potential of the magnetic layer were obtained. An arithmetical mean of the obtained three values was shown in Table 1, as the isoelectric point of the surface zeta potential of the magnetic layer of each magnetic tape. As a surface zeta potential measurement device, SurPASS manufactured by Anton Paar was used. The measurement conditions were set as follows. Other details of the method of obtaining the isoelectric point is as described above.

Measurement cell: variable gap cell (20 mm×10 mm)

Measurement mode: Streaming Current

Gap: approximately 200 μm

Measurement temperature: room temperature

Ramp Target Pressure/Time: 400,000 Pa (400 mbar)/60 seconds Electrolyte: KCl aqueous solution having concentration of 1 mmol/L (adjusted pH to 9)

pH adjusting solution: HCl aqueous solution having concentration of 0.1 mol/L or KOH aqueous solution having concentration of 0.1 mol/L

Measurement pH: pH 9→pH 3 (measured at 13 measurement points in total at interval of approximately 0.5

(2) Measurement of Decrease in Electromagnetic Conversion Characteristics During Repeated Reproducing in Low Temperature and Low Humidity Environment

The following measurement was performed in the low temperature and low humidity environment of a temperature of 10° C. and relative humidity of 10%.

A decrease in Signal-to-Noise Ratio (SNR) during repeating reproducing was measured by the following method using a reel tester having a width of ½ inches (0.0127 meters) to which a recording head is fixed.

A relative speed of a magnetic tape and a magnetic head was set as 8 m/sec, a metal-in-gap (MIG) head (gap length of 0.15 μm, track width of 1.0 μm) was used as a magnetic head for recording, and a recording current was set as an optimal recording current of each magnetic tape. As a magnetic head (reproducing head) for reproducing, a giant-magnetoresistive (GMR) head having an element thickness of 15 nm, a shield interval of 0.1 μm, and lead width of 0.5 μm was used. A signal having linear recording density of 300 kfci was recorded and a reproducing signal was measured with a spectrum analyzer manufactured by Shibasoku Co., Ltd. A part of a signal which is sufficiently stabilized after starting the running of the magnetic tape was used.

Under the conditions described above, the magnetic tape and the reproducing head were slid on each other for 2,000 passes, with 1,000 m per 1 pass, to perform the reproducing. A ratio of an output value of a carrier signal and integral noise over whole spectral range was set as an SNR. The SNR (Broadband-SNR (BB-SNR)) was obtained by using a relative value in a case where an SNR of Comparative Example 1 for 1 pass was set as reference (0 dB). A difference “(SNR of 2,000-th pass)—(SNR of first pass)” was shown in Table 1 as a decrease in SNR during repeated reproducing.

The results of the above measurement are shown in Table 1 (Table 1-1 and Table 1-2).

TABLE 1-1 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Formation of Content of Binding    5 parts   10 parts   15 parts   20 parts    0 parts   10 parts magnetic layer binding agent agent A in magnetic Binding    0 parts    0 parts    0 parts    0 parts   10 parts   10 parts liquid agent B Alternating magnetic field Performed Performed Performed Performed Performed Performed application during coating Burnishing process Performed Performed Performed Performed Performed Performed Isoelectric point of surface zeta potential of 5.5 6.1 6.3 6.5 6.0 6.4 magnetic layer Decrease in SNR −0.3 dB −0.5 dB −0.5 dB −0.4 dB −0.5 dB −0.5 dB

TABLE 1-2 Comparative Comparative Comparative Comparative Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Formation Content of Binding    5 parts   10 parts   15 parts   20 parts   15 parts   15 parts    0 parts   10 parts of binding agent A magnetic agent in Binding    0 parts    0 parts    0 parts    0 parts    0 parts    0 parts   10 parts   10 parts layer magnetic agent B liquid Alternating Not Not Not Not Performed Not Not Not magnetic field performed performed performed performed performed performed performed application during coating Burnishing process Not Not Not Not Not Performed Not Not performed performed performed performed performed performed performed Isoelectric point of surface zeta 5.0 4.8 4.6 4.6 4.3 4.6 4.8 4.7 potential of magnetic layer Decrease in SNR −5.1 dB −2.5 dB −2.5 dB −3.8 dB −2.3 dB −2.5 dB −2.4 dB −2.5 dB

From the results shown in Table 1, in the magnetic tapes of the examples in which the isoelectric point of the surface zeta potential of the magnetic layer is equal to or greater than 5.5, it is possible to confirm that a decrease in electromagnetic conversion characteristics (SNR) during repeated reproducing in the low temperature and low humidity environment is prevented, compared to that in the magnetic tapes of the comparative examples.

One aspect of the invention is effective in a technical field of various magnetic recording medium such as a magnetic tape for data storage. 

What is claimed is:
 1. A magnetic recording medium comprising: a non-magnetic support; and a magnetic layer including ferromagnetic powder and a binding agent on the non-magnetic support, wherein the isoelectric point of the surface zeta potential of the magnetic layer is equal to or greater than 5.5.
 2. The magnetic recording medium according to claim 1, wherein the isoelectric point is 5.5 to 7.0.
 3. The magnetic recording medium according to claim 1, wherein the binding agent is a binding agent including an acidic group.
 4. The magnetic recording medium according to claim 3, wherein the acidic group includes at least one kind of acidic group selected from the group consisting of a sulfonic acid group and a salt thereof.
 5. The magnetic recording medium according to claim 2, wherein the binding agent is a binding agent including an acidic group.
 6. The magnetic recording medium according to claim 5, wherein the acidic group includes at least one kind of acidic group selected from the group consisting of a sulfonic acid group and a salt thereof.
 7. The magnetic recording medium according to claim 1, further comprising: a non-magnetic layer including non-magnetic powder and a binding agent between the non-magnetic support and the magnetic layer.
 8. The magnetic recording medium according to claim 1, further comprising: a back coating layer including non-magnetic powder and a binding agent on a surface side of the non-magnetic support opposite to a surface side provided with the magnetic layer.
 9. The magnetic recording medium according to claim 1, wherein the isoelectric point of the surface zeta potential of the magnetic layer is 5.5 to 6.5. 